Optical substrates, such as objective lenses in optical pickup apparatuses and semiconductor apparatuses, spectacle lenses, optical reflecting mirrors, lowpass filters, etc., are provided with anti-reflection coatings to improve light transmittance. The anti-reflection coatings have conventionally been formed by a physical method such as vacuum vapor deposition, sputtering, ion plating, etc. However, these film-forming methods are expensive because a vacuum equipment is needed.
A single-layer anti-reflection coating is designed to have a refractive index smaller than that of a substrate and larger than that of an incident medium such as air, etc. It is considered ideal for an anti-reflection coating formed on a glass lens having a refractive index of about 1.5 to have a refractive index of 1.2-1.25. However, there is no material having such an ideal refractive index, which can be formed into an anti-reflection coating by a physical method. Accordingly, MgF2 having a refractive index of 1.38 is widely used for anti-reflection coatings.
Because optical equipments using light rays in a wide wavelength region have recently been put into practical use, anti-reflection coatings having excellent optical characteristics in a wide wavelength range have become desired. Because many optical members are constituted by pluralities of lenses, transmission loss by reflection on each lens surface should be suppressed as much as possible. For this purpose, multilayer anti-reflection coatings are generally used. A multilayer anti-reflection coating is designed such that light rays reflected at each interface and those entering each layer are canceled by interference. However, the multilayer anti-reflection coatings are disadvantageous in high cost.
Thus proposed is a method for forming an anti-reflection coating by a wet method (a dip-coating method, a roll-coating method, a spin-coating method, a flow-coating method, a spray-coating method, etc.) utilizing a sol-gel method comprising dehydration polycondensation.
For instance, JP 2006-215542 A proposes an anti-reflection coating comprising a dense coating and a porous silica aerogel layer formed in this order on a substrate, which has a refractive index decreasing from the substrate to the porous silica aerogel layer in this order. This porous silica aerogel layer is formed by (i) reacting a silicon oxide sol or gel with an organic-modifying agent to form an organically modified sol or gel, (ii) coating the organically modified sol or a sol formed from the organically modified gel onto a dense coating, the resultant organically modified silica gel layer being turned to an organically modified silica aerogel layer by a springback phenomenon, and (iii) heat-treating the organically modified silica aerogel layer to remove organic-modifying groups.
The porous silica aerogel layer has as small a refractive index as about 1.20, and an anti-reflection coating having such porous silica aerogel layer has excellent anti-reflection characteristics in a wide wavelength range. In addition, because the porous silica aerogel layer can be formed by a sol-gel method, it has excellent cost performance. However, the porous silica aerogel layer has small mechanical strength and adhesion to a substrate, and insufficient scratch resistance.
JP 2006-130889 A proposes a transparent, porous, inorganic coating free from cracking and peeling due to shrinkage occurring by a heat treatment despite its thickness of more than 1 μm, and having a small refractive index, and high transmittance of 90% or more in a region from visible light to near infrared light, which is a thin, mesoporous silica layer formed on a substrate and having nano-scale micropores. This thin, mesoporous silica layer is formed by coating a mixed solution comprising a surfactant, a silica-forming material such as tetraethoxysilane, water, an organic solvent, and acid or alkali onto a substrate to form an organic-inorganic composite coating, drying this coating, and photo-oxidizing it to remove organic components.
Japanese Patent 3668126 proposes a method for forming a ceramic layer having a low dielectric constant (high porosity and low refractive index), by preparing a liquid comprising a ceramic precursor such as tetraethoxysilane, a catalyst, a surfactant and a solvent, coating the liquid onto a substrate, and removing the solvent and the surfactant to form a porous silica layer.
However, the thin, mesoporous silica layer of JP 2006-130889 A and the porous silica layer of Japanese Patent 3668126 are insufficient in a balance of anti-reflection performance, a refractive index, scratch resistance, adhesion to a substrate, mechanical strength and moisture resistance. In addition, because the thin, mesoporous silica layer of JP 2006-130889 A and the porous silica layer of Japanese Patent 3668126 are obtained by forming a silicate network around a surfactant micelle during drying the coating, and proceeding the hydrolysis and polycondensation of the silicate to turn the network to a thin solid layer, they are non-uniform, with their production taking a long period of time for coating, hydrolysis and polycondensation.
“Chemical Industries,” September, 2005, Vol. 56, No. 9, pp. 688-693, issued by Kagaku Kogyo-Sha, describes a coating of mesoporous silica nano-particles having high light transmittance, which is obtained by aging a mixture solution comprising tetraethoxysilane, a cationic surfactant (cetyltrimethyl ammonium chloride) and a nonionic surfactant represented by [HO(C2H4O)106—(C3H6O)70—(C2H4O)106H] under an acidic condition in the presence of hydrochloric acid; further aging it with ammonia water to prepare a solution of mesoporous silica nano-particles covered with the nonionic surfactant and containing the cationic surfactant in pores; coating this solution onto a substrate; drying the resultant coating; and baking it to remove the cationic surfactant and the nonionic surfactant. However, this reference fails to teach the use of such coating of mesoporous silica nano-particles as an anti-reflection coating.